Injection-molded products include a molded product composed of a unified laminate of a plurality of resin layers (hereinafter referred to as a molded laminate product), for example, a molded product composed of resin layers of a combination of various colors and materials, or a molded product composed or a laminate of resin layers of the same material.
Main conventional molding methods for producing such a molded laminate product include a multicolor molding method and an insert molding method. The multicolor molding method involves injecting a plurality of resin materials from different injection apparatuses into a cavity in a mold, thereby laminating a plurality of resin layers. The insert molding, method involves placing an injection-molded semifinished product as an insert in a cavity in a mold, and then injecting a resin material into the cavity to mold a laminate.
These days optical parts, such as a plastic lens, are commonly produced by injection molding. While it is conventional practice to mold a plastic lens as a single-layer lens, there exist some thick plastic lenses, such as a light diffusing lens for use in outdoor lighting, an automotive headlight, etc.
Such a thick light diffusing lens is required to be a homogeneous transparent body free of optical defects so that light will diffuse uniformly. In the case of molding a lens composed of a laminate of a plurality of resin layers, differences may be produced in physical properties, such as residual stress and internal distortion, between the resin layers even when the same molding material is used for the layers; therefore, it is difficult to mold a high-quality lens. Especially in the case of multicolor molding of a laminate, a difference will be produced in thermal history between the resin layers. Therefore, multicolor molding is not suited to molding of a thick lens such as a light diffusing lens. Thus, at present, a light diffusing lens is usually produced as a single-layer molded product when the lens is produced by injection molding.
However, in the case of injection molding of a single-layer light diffusing lens, defects such as sinks are likely to be formed in a thick portion. Therefore, in a pressure-keeping process, it is necessary to cool a molded product slowly over time. Accordingly, it takes a long time to mold such a lens.
In order to shorten the time for molding a lens and increase the production efficiency, it is presently being studied to mold a lens, composed of not a single layer but a laminate of a plurality of layers, by employing the insert molding method.
For example, PCT International Publication No. WO 2012/132597 has proposed a method which involves forming a plurality of cavities having different volumes in a mold, and repeating injection of a molten resin while sequentially transferring an intermediate molded product as an insert to a cavity having a larger volume than the intermediate molded product, thereby producing a molded laminate product.